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Power System Protection
Rizwan KhanUniversity of Engineering and Technology, Lahore
1
Transformer Protection
2Rizwan Khan (Lecturer - UET LHE)
Introduction
Overcurrent Protection
Percentage Differential Protection
Causes of False Differential Currents
Supervised Differential Relays
Three Phase Transformer Protection
Contents of Transformer Protection
3Rizwan Khan (Lecturer - UET LHE)
Volts per Hertz Protection
Non Electrical Protection
Protection Systems for Transformers
Summary
4
Contents of Transformer Protection
Rizwan Khan (Lecturer - UET LHE)
5Rizwan Khan (Lecturer - UET LHE)
Transformer Faults Statistics
6Rizwan Khan (Lecturer - UET LHE)
The inherent characteristics of power transformers introduce a
number of unique problems that are not present in the protection
of transmission lines, generators, motors or other power system
apparatus.
In general, a transformer may be protected by fuses, overcurrent
relays, differential relays and pressure relays, and can be
monitored for incipient trouble with the help of winding
temperature measurements, and chemical analysis of the gas
above the insulating oil.
Introduction
7Rizwan Khan (Lecturer - UET LHE)
Transformer Size
Location and Function
Voltage
Connection and Design
Factors of Transformer Protection
8Rizwan Khan (Lecturer - UET LHE)
Transformer Size
9
Transformer Size Protection
2500kVAFuses
Thermal Overload Relays
2500-5000kVAFuses
Overcurrent Relays
5000-10000kVA
Fuses
Overcurrent Relays
Ordinary Differential Relays
Above 10MVA
Fuses
Overcurrent Relays
Percentage Differential Relays
Temperature & Pressure Relays
If the transformer is an integral part of the bulk power system,
it will probably require more sophisticated relays in terms of
design and redundancy.
If it is a distribution station step down transformer, a single
differential relay and overcurrent backup will usually sufficient.
If the transformer is near a generation source, the high X/R ratio
of the fault path will require harmonic restraint relays to
accommodate the higher magnetic inrush currents.
Location and Function
10Rizwan Khan (Lecturer - UET LHE)
Generally, higher voltages demand more sophisticated and costly
protective devices, due to the effect of a delayed fault clearing on
the system performance, and the high cost of transformer repair.
The protection schemes will vary considerably between auto
transformers, and two or three winding transformers. The
winding connection of a three phase transformer, whether delta or
wye, will make a difference in the protection scheme chosen.
Also important are the presence of tertiary windings, type of
grounding used, tap changers or phase-shifting windings.
Voltage, Connection and Design
11
12Rizwan Khan (Lecturer - UET LHE)
Fuses
Time Delay Overcurrent Relays
Instantaneous Relays
Overcurrent Protection
13Rizwan Khan (Lecturer - UET LHE)
Clearly, the fuse interrupting capability must exceed the maximum
short-circuit current that the fuse will be called upon to interrupt.
The continuous rating of the fuse must exceed the maximum
transformer load.
Typically, the fuse rating should be greater than 150% of the
maximum load.
It is also clear that the transformer magnetizing current should not
cause damage to the fuse.
Fuse
14Rizwan Khan (Lecturer - UET LHE)
The minimum melt characteristic of the fuse must coordinate with
(i.e. should be well separated from) the protective devices on the
low side of the power transformer. In considering the
coordination, the ambient temperature, prior loading and reclosing
adjustment factors should be taken into account.
The speed ratio of the fuse is dened as the ratio between the
minimum melt current values at two widely separated times: for
example, 0.1 and 100 s. It is desirable to have a fuse with as high a
speed ratio as possible.
Fuse
15Rizwan Khan (Lecturer - UET LHE)
Fuse
16
Protection against excessive overload, or persisting external fault,
is provided by time-delay overcurrent relays.
The pickup setting is usually 115% of the maximum overload
acceptable.
The time-delay overcurrent relays must coordinate with the low-
side protective devices. These may include low-voltage bus relays
for phase-to-phase faults, phase-directional relays on parallel
transformers and the breaker failure relay timers on the low-
voltage breakers.
Time Delayed/Instantaneous O/C Relays
17Rizwan Khan (Lecturer - UET LHE)
18Rizwan Khan (Lecturer - UET LHE)
Differential Protection
19Rizwan Khan (Lecturer - UET LHE)
For a normal power transformer
If we use current transformers having turns ratios of 1 : n1 and 1 :
n2 on the primary and the secondary side respectively, under
normal conditions the currents in the secondary windings of the
current transformers are related by
20
Differential Protection
Rizwan Khan (Lecturer - UET LHE)
If we select the CTs appropriately, we may make N1n1 = N2n2, and
then, for a normal transformer, i1s = i2s. However, if an internal
fault develops, this condition is no longer satisfied, and the
difference of ils and i2s becomes much larger; in fact, it is
proportional to the fault current. The differential current provides a
highly sensitive measure of the fault current.
21
Differential Protection
Rizwan Khan (Lecturer - UET LHE)
Mismatching in CT Ratings
Mismatching in Errors of Transformations
Tap Changer of Transformer
22
Practical Issues in Differential Protection
Rizwan Khan (Lecturer - UET LHE)
First, it may not be possible to obtain the CT ratios on the primary
and the secondary side which will satisfy the condition N1n1 =
N2n2, as we must select CTs with standard ratios.
A somewhat less desirable procedure is to use auxiliary CTs to
achieve the same goal.
In any case, even with these adjustments, there remains some
residual ratio mismatch, which leads to a small differential current
id during normal conditions.
23
Practical Issues in Differential Protection
Rizwan Khan (Lecturer - UET LHE)
Second, the errors of transformation of the two CTs may differ
from each other, thus leading to significant differential current
when there is normal load flow, or an external fault.
Finally, if the power transformer is equipped with a tap changer, it
will introduce a main transformer ratio change when the taps are
changed.
24
Practical Issues in Differential Protection
Rizwan Khan (Lecturer - UET LHE)
In a percentage differential relay, the differential current must exceed a
fixed percentage of the through current in the transformer. The through
current is defined as the average of the primary and the secondary
currents:
The current ir is known as the restraint current a name that comes from
the electromechanical relay design, where this current produced a
restraint torque on the moving disc25
Percentage Differential Protection
26
Percentage Differential Relay Characteristics
Rizwan Khan (Lecturer - UET LHE)
27Rizwan Khan (Lecturer - UET LHE)
Magnetizing Inrush Current during Energization
Harmonic Content of the Inrush Current
Magnetizing Inrush during Fault Removal
Sympathetic Inrush
Transformer Over-excitation
CT Saturation 28
Causes of False Differential Currents
Rizwan Khan (Lecturer - UET LHE)
Consider the energization of an unloaded power transformer. As
the switch is closed, the source voltage is applied to the
transformer, and a magnetizing current is drawn from the source.
Let the source voltages and flux linkages are,
29
Magnetizing Inrush Current during Energization
Rizwan Khan (Lecturer - UET LHE)
30
Magnetizing Inrush Current during Energization
Rizwan Khan (Lecturer - UET LHE)
In reality, the magnetizing inductance of the transformer is
nonlinear.
As the flux linkages go above the saturation knee point, a much
larger current is drawn from the source. The magnitude of this
current is determined by the slope of the magnetizing
characteristic in the saturated region, and by the leakage
inductance of the transformer.
It is obvious that magnetizing inrush currents of the order of fault
currents are possible.31
Magnetizing Inrush Current during Energization
Rizwan Khan (Lecturer - UET LHE)
It should be clear that in most modern transformers very large
inrush currents are possible, depending upon the instant of
energization, and the remnant flux in the transformer core. Since
the inrush current flows only in the primary and not in the
secondary winding of the transformer, it is clear that it produces a
differential current which is 200% of the restraining current, and
would cause a false operation.
32
Magnetizing Inrush Current during Energization
Rizwan Khan (Lecturer - UET LHE)
The false operation of a percentage differential relay for a
transformer is prevented by taking advantage of the fact that the
inrush current is rich in harmonic components.
While the fault current is a pure fundamental frequency
component (except for a possible decaying DC component).
33
Harmonics Content of the Inrush Current
Rizwan Khan (Lecturer - UET LHE)
When a fault external to, but near, the transformer is removed by
the appropriate circuit breaker, the conditions inside the
transformer core are quite similar to those during magnetization of
the transformer.
As the voltage applied to the transformer windings jumps from a
low pre-fault value to the normal (or larger) post-fault value, the
flux linkages in the transformer core are once again forced to
change from a low pre-fault value to a value close to normal.
34
Magnetizing Inrush Current during Fault Removal
Rizwan Khan (Lecturer - UET LHE)
Depending upon the instant at which the fault is removed, the
transition may force a DC offset on the flux linkages, and primary
current waveforms similar to those encountered during
energization would result.
It should be noted that as there is no remnant flux in the core
during this process; the inrush is in general smaller than that
during the transformer energization.
35
Magnetizing Inrush Current during Fault Removal
Rizwan Khan (Lecturer - UET LHE)
36
Sympathetic Inrush
Rizwan Khan (Lecturer - UET LHE)
The phenomenon which causes inrush to flow in a previously
energized transformer, when a parallel bank is energized, is known
as the sympathetic inrush.
The current waveforms of a typical sympathetic inrush
phenomenon are illustrated in next slide.
37
Sympathetic Inrush
Rizwan Khan (Lecturer - UET LHE)
38
Sympathetic Inrush
During load rejection and certain other operating conditions, a
transformer may be subjected to a steady-state overvoltage at its
nominal frequency.
During over-excitation, the transformer flux remains symmetric,
but goes into saturation for equal periods in the positive and the
negative half-periods of the waveform, due to the rich contents of
the harmonics.
39
Transformer Over-excitation
Rizwan Khan (Lecturer - UET LHE)
For certain external faults, where the fault currents are large, it is
likely that one of the CTs may saturate, this may directly effect the
operation of both ordinary and percentage differential relay.
40
CT Saturation
Rizwan Khan (Lecturer - UET LHE)
41Rizwan Khan (Lecturer - UET LHE)
To desensitize the differential relay when the transformer is
energized.
Introduction of the Concept of Voltage Supervision
Harmonics Characterization as Supervised Differential Relay
42
How to Avoid False Differential Currents
Rizwan Khan (Lecturer - UET LHE)
One of the earliest ideas was to desensitize the differential relay when
the transformer is energized. Thus, the less sensitive relay would not see
the inrush current, thus avoiding a false trip.
However, this is a poor practice, as it is precisely during the initial
energization of the transformer, when the transformer is first energized,
or some repair work on the transformer may have been completed, that
the transformer is in need of protection. This is to ensure that the repair
work has been successfully completed, and no maintenance tools
inadvertently left inside or around the transformer.
43
Desensitize the Differential Relay
Rizwan Khan (Lecturer - UET LHE)
It may be expected that during the inrush conditions, the
transformer voltage would be close to normal, while during faults,
the voltage would be much less. Thus, an under-voltage relay may
be used to supervise the differential relay.
In general, this type of voltage supervision is not preferred, as the
under-voltage relay tends to be slow, and consequently the entire
protection becomes slower.
44
Voltage Supervision
Rizwan Khan (Lecturer - UET LHE)
The method currently in use on large transformers is based upon using
the harmonic characterization of the inrush and over-excitation currents.
The differential current is almost purely sinusoidal when the transformer
has an internal fault, whereas it is full of harmonics when the
magnetizing inrush current is present, or when the transformer is
overexcited.
Thus, the differential current is filtered with filters tuned to an
appropriate set of harmonics, and the output of the filters is used to
restrain the differential relay.45
Harmonics Characterization
46
Supervised Differential Relays
47Rizwan Khan (Lecturer - UET LHE)
The major difference between three-phase transformer protection and
that of three single-phase transformers is the necessity to deal with the
effect of a wyedelta transformation.
Under normal load conditions, the currents in the primary and secondary
windings are in phase, but the line currents on the wye and delta sides of
the three-phase transformer are out of phase by 30.
The difficulty is resolved by connecting the current transformers in such
a manner that they undo the effect of the wyedelta phase shift produced
by the main transformer.48
Three Phase Transformer Protection
Rizwan Khan (Lecturer - UET LHE)
The current transformers on the wye side of the power transformer
are connected in delta, and the current transformers on the delta
side of the power transformer are connected in wye.
In addition to the phasing consideration discussed above, it is also
necessary to adjust the turns ratios of the CTs so that the delta
connection on the wye side of the power transformer produces
relay currents that are numerically matched with the relay currents
produced by the wye-connected CTs.
49
Three Phase Transformer Protection
Rizwan Khan (Lecturer - UET LHE)
50
Three Phase Transformer Protection
Consider the three-winding transformer shown in Figure 8.13. One
winding is assumed to be delta connected, while the other two are
assumed to be wye connected. The CTs must of course be
connected in wye on the delta side and in delta on the wye side of
the power transformer.
This will ensure that the phase shifts created in the currents of the
power transformer are compensated by the CTs, so that the
secondary currents are once again in phase.
51
Multi Winding Transformer Protection
Rizwan Khan (Lecturer - UET LHE)
52
Multi-winding Transformer Protection
Rizwan Khan (Lecturer - UET LHE)
It is interesting to note that under certain conditions a two-winding
differential relay can be used to protect a three-winding
transformer. If the transformer is connected to a source only on
one side, the other two winding CTs could be paralleled to
produce a net secondary current, which can then be used in a two-
winding protection scheme.
53
Multi Winding Transformer Protection
Rizwan Khan (Lecturer - UET LHE)
The regulating transformers may regulate the turns ratio or the
phase shift between the primary and the secondary windings.
The regulating transformers usually consist of two transformers:
one to provide the magnetizing current for the transformers and
the other to provide the variable turns ratio or the variable phase
shift.
In either case, the percentage differential relay is not suitable for
the protection of these transformers. Often, a sudden pressure
relay (SPR) provides the most sensitive protection.54
Regulating Transformers Protection
55Rizwan Khan (Lecturer - UET LHE)
Transformer cores are normally subjected to flux levels
approaching the knee point in their magnetizing characteristic.
If the core flux should exceed the saturation level, the flux patterns
in the core and the surrounding structure would change, and
significant flux levels may be reached in the transformer tank and
other structural members. As these are not laminated, very high
eddy currents are likely to result, producing severe damage to the
transformer.
56
Voltage/Hertz Protection
Rizwan Khan (Lecturer - UET LHE)
As the flux is proportional to the voltage, and inversely
proportional to the operating frequency, the significant relaying
quantity is the ratio of the per unit voltage to the per unit
frequency. This is known as volts/hertz protection.
This protection is specially needed in the case of unit-connected
generator transformers. If the turbinegenerator is shut down with
the voltage regulator in service, the volts/hertz limit of the
transformers (and indeed of generators as well) could be easily
exceeded.57
Voltage/Hertz Protection
Rizwan Khan (Lecturer - UET LHE)
Similar conditions could also be reached by load rejection with
voltage regulators disconnected, or in manual position, or with
faulty instrumentation in the regulator circuits.
The volts/hertz capability of transformers is specified by
manufacturers.
Many volts/hertz relays have two settings, a lower setting for
alarm and a higher setting which may be used for tripping.
58
Voltage/Hertz Protection
Rizwan Khan (Lecturer - UET LHE)
59
Voltage/Hertz Protection
Rizwan Khan (Lecturer - UET LHE)
60Rizwan Khan (Lecturer - UET LHE)
When a fault occurs inside an oil-filled transformer tank, the fault
arc produces gases, which create pressure waves inside the oil. In
the conservator type of tank construction, which is more
common in Europe, the pressure wave created in the oil is detected
by a pressure vane in the pipe which connects the transformer tank
with the conservator.
The movement of the vane is detected by a micro-switch, which
can be used to sound an alarm, or trip the transformer. This type of
a relay is known as a Buchholz relay.61
Pressure Devices
In the USA, the more common transformer construction is of the
tank type with a gas cushion at the top of the tank. In such a
transformer, the pressure wave is detected by a SPR mounted on
the side of the transformer. The mechanical sensor consists of a
bellows and a pressure equalizer, which together is insensitive to
slow changes of pressure, for example those caused by thermal or
loading changes in the transformer.
However, a pressure wave created by a fault is detected by the
relay, and can be used to trip or alarm.62
Pressure Devices
Faults on the bushings, and connecting leads, do not create an arc
in the insulating oil or gas, and must be protected by differential
relays.
63
Pressure Devices
Rizwan Khan (Lecturer - UET LHE)
Some devices simply measure the oil temperature, usually the top
oil.
Other devices use a combination of current, by placing a small
search coil around a lead, and oil temperature to measure the total
effect of load and ambient temperature.
The critical temperature is referred to as the hot-spot
temperature, and is the highest temperature that will occur
somewhere in the winding.
64
Temperature Devices
Rizwan Khan (Lecturer - UET LHE)
The temperature devices actuate alarms to a central dispatching
office, to alert the operators, who can either remotely unload the
transformer by opening the circuit breaker, or can dispatch an
operator to the station.
The hot-spot sensors are also commonly used to start and stop
cooling fans and pumps.
65
Temperature Devices
Rizwan Khan (Lecturer - UET LHE)
66Rizwan Khan (Lecturer - UET LHE)
Parallel Transformer Banks
Tapped Transformer Banks
Substation Design
Station Service
Generator Station Design
67
Protection Systems for Transformers
Rizwan Khan (Lecturer - UET LHE)
It is not uncommon when two transformers are connected in
parallel with no circuit breakers used to separate them that a single
differential relay is used for the protection of both banks.
However, the problem of sympathetic inrush presents difficulties
to the common differential relay, when one of the transformers is
energized with its switch while the other transformer is in service.
The sympathetic inrush is now likely to trip the two transformers.
The problem can be dealt with satisfactorily by the use of separate
differential relays.68
Parallel Transformer Banks
Rizwan Khan (Lecturer - UET LHE)
Connecting a transformer directly to the high-voltage transmission
line is a practical and relatively inexpensive way of providing
service to an isolated industrial or residential load.
In the next slides, complete protection schemes are introduced for
the protection of lines and for the protection of tapped transformer,
in different scenarios.
69
Tapped Transformer Banks
Rizwan Khan (Lecturer - UET LHE)
70
Tapped Transformer Banks
Full complement of circuit breakers for transformer protection
71
Tapped Transformer Banks
Direct connection of transformer and transmission lines - shows a variation in which
the line circuit breakers are not used. In this configuration a line fault will remove the
transformer by opening the remote terminals.
72
Tapped Transformer Banks
Use of motor-operated air break switches at a transformer tap
Since a power transformer interconnects two or more voltage
levels, its location requires special consideration in the design of a
substation and the protection of all of the elements within it.
Usually the various voltage levels are contained within separate
areas, each with its own bus configuration and associated
equipment and separated by considerable distances.
Proceeding Figures show 345 kV/138 kV station with different
protection schemes.
73
Substation Design
Rizwan Khan (Lecturer - UET LHE)
74
Substation Design
Substation transformer with a full complement of circuit breakers
75
Substation Design
MOAB connection of a transformer to a 345 kV bus
Most power transformers are built with a delta tertiary winding to
provide a path for third harmonics and to help stabilize the neutral.
Although the capacity of the tertiary need not be very large, it is a
good source for the station auxiliary equipment such as circuit
breaker air compressors, oil pumps, battery chargers, etc.
If the load taken off the tertiary is large enough to upset the
transformer differential, it must be connected into the differential
CTs, as shown in Figure on next slide.
76
Station Service
Rizwan Khan (Lecturer - UET LHE)
77
Station Service
Protection of generating station service transformer
The most common configuration used presently for large
generating plants is a unit-connected system. In this system, the
generator, generator step up transformer (GSU), the unit auxiliary
transformer (UAT) and the reserve or start up auxiliary
transformer (RAT) are all connected as a unit and their protection
and controls are interrelated.
Figure on next slide, shows a typical design with the associated
zones of protection.
78
Generator Station Design
Rizwan Khan (Lecturer - UET LHE)
79Generating station transformers and their protection systems
References
Stanley H. Horowiz, Arun G. Phadke, Power System
Relaying, 3rd Edition, Chapter No. 8
THANKS FOR YOUR TIME81